One example of the conventionally-known needle bar drive apparatus for sewing machines is disclosed in Japanese Patent Publication No. HEI-3-37960, which particularly shows an embroidery sewing machine equipped with needle bar driving mechanisms driven by linear motors. According to the disclosure, a separate linear motor for driving a needle bar is provided for each machine head of the multi-head embroidery sewing machine, and a detection device is provided for detecting a rotational angle of a hook shaft that rotates a rotary hook provided per machine head. Because operation of each of the linear motors is controlled in synchronism with the rotational angle of the hook shaft detected by the detection device, the disclosed technique can eliminate a need for a complicated power transmission mechanism, such as a cam mechanism, for driving the needle bar, and it can simplify the construction of the needle bar driving mechanism. Further, by appropriately controlling the behavior or condition of the linear motor, the disclosed technique can freely set the needle bar driving timing in response to a change in the sewing condition.
According to the disclosure in the No. HEI-3-37960 publication, a stroke over which the needle bar is driven to move up and down during a sewing operation has a fixed length, and the top dead center of the stroke is set at a predetermined position greatly spaced apart from the upper surface of a machine table. This is because there has been a need to secure a sufficient interval or space between the lower end of the needle bar positioned at the top dead center that corresponds to the above-mentioned predetermined position, in order to avoid various inconveniences, such as the inconvenience that the lower end of the needle bar undesirably contacts an object to be sewn (fabric or cloth) during an operation for changing the cloth positioned on the machine table. Namely, it has been necessary that the stroke length over which the needle bar is driven to move up and down be set greater than a necessary minimum length. With such arrangements disclosed in the No. HEI-3-37960 publication, there would be encountered the inconvenience that noise and vibration tends to be great during the sewing operation due to the great stroke length of the needle bar. Further, because the needle bar stroke has a great fixed length, freedom with which to set the drive timing of the needle bar also tends to be limited. However, with diversification of embroidering operations today, there is also a demand that a space between the lower end of the needle bar in a rest (i.e., non-driven) state and the upper surface of the machine table be made as great as possible.